While you're spending money, why not get a better computer, too? If you have lots of users or lots of data (or both), your database server needs all the power you can give it. After all, FileMaker Server performs finds, edits records, sorts, imports, exports, and otherwise constantly busies itself with the work of every user.
The most important thing you can do to make FileMaker Server faster is give it faster access to the data on the disk. At the very least, this increased speed means you should never store the files on a file server. Instead, they should always be on a hard drive in the host computer. If you still find things aren't as fast as you'd like them, your best upgrade is often a faster disk system—something called a RAID, or Redundant Array of Inexpensive Disks. You can ignore the "Redundant" part and the "Inexpensive" part, though. A RAID is an array of hard disks. By putting several disks together, you can get faster access to your data than you would with just one disk. As important, you can get more reliable access as well, since the same data can be stored in more than one place in case a disk drive fails, as outlined in the box on the next page.
In addition to a fast disk system, you should consider adding plenty of RAM to the host computer. The least important component is the CPU speed itself (but that's not to say it doesn't matter; don't expect a fast server with a 500 MHz processor).
If you find all this terribly confusing, be comforted by the ...